How to animate text being revealed on the screen
From my experience text objects do not work very well with booleans (due to some letters in some fonts being non-manifold).

This is a method that can be adapted to work on anything:


You need to create a gradient mask for your material that will "stay static" despite the object movement (you do that by using object mapping and adding/subtracting the object's position from it):



link to full image

Great thing about a mask is that it can be faded on the interface.
The Add node is just driven by the text position.x.
The mask is used to mix Transparent and any other shader. 
If you need to place such Text somewhere you need to parent it to something first and move it through parent.


This is the result:



And the blend file: